This invention relates to the manufacturing technology of superalloys, and, more particularly, to the prevention or reduction of quench cracking of superalloys that are quenched during their processing.
Superalloys are metallic alloys developed for high-temperature service under extreme conditions including high loading, fatigue, thermal gradients, oxidation, and corrosion. The commercially most important of the superalloys are nickel-base and cobalt-base alloys used in aircraft gas turbine applications. Such superalloys are used in cast parts such as turbine blades and vanes, and in wrought parts such as turbine disks. The present invention relates to the manufacturing technology of wrought superalloys.
A wrought article is usually prepared by furnishing a blank of the superalloy material, and deforming the blank by a metal-working process such as forging to form a preform. In most cases, the preform is thereafter heated to elevated temperature to attain a particular microstructure and then cooled rapidly ("quenched") to lower temperature to retain that structure. The article is then reheated to a lower temperature.
Some of the most important and most advanced superalloys are prone to cracking during the quenching operation. Such behavior is generally known as quench cracking. Quench cracks appear at the surface of the article, either throughout the surface or at crack-prone regions. Quench cracks are of great concern. If allowed to remain on the article, the quench cracks can eventually lead to premature failure of the article, usually by fatigue crack propagation from the quench cracks. Quench cracking of wrought superalloys is therefore a problem of great concern in aircraft gas turbine manufacturing.
It is difficult to predict which superalloys will be prone to quench cracking, or the extent to which any particular superalloy may quench crack during processing. Generally, however, if a superalloy article of a particular configuration exhibits quench cracks after being processed in an otherwise desirable manufacturing sequence, it is said to be prone to quench cracks.
The propensity for quench cracking is influenced by many variables, including the composition of the alloy, its microstructure, its mechanical and physical properties, the quenching medium, the temperature from which the material is quenched, part size and configuration, especially such design factors as sharp corners and abrupt changes in section size. For example, a particular superalloy may exhibit quench cracks when quenched in water or oil, but not when quenched in moving air. If the manufacturing operation requires an air quench to achieve a desired microstructure of the article, then this particular superalloy would not be prone to quench cracking. On the other hand, if the manufacturing operation requires a water or oil quench to achieve a desired microstructure, this superalloy would be prone to quench cracking. If the quenching rate is sufficiently high, then virtually any superalloy could exhibit quench cracking. Similarly, a particular superalloy formed into one shape may exhibit quench cracking, but not when formed into a different shape.
Thus, those skilled in the art of wrought superalloy manufacturing technology recognize which superalloys are prone to quench cracking in various situations, usually by observing quench cracking under particular conditions. Stronger, less ductile alloys usually show the greatest inclination to quench cracking. Some of the advanced superalloys especially developed for service at high temperatures contain large amounts of gamma prime, and are particularly susceptible to quench cracking. An example of a superalloy that is prone to quench cracking when solutioned above the gamma-prime solvus temperature is Rene'95, which has a nominal composition, in weight percent, of 14% Cr, 8% Co, 3.5% Mo, 3.5% W, 3.5% Nb, 2.5% Ti, 3.5% Al, 0.15% C, 0.01% B, 0.05% Zr, balance Ni and incidental impurities.
There is therefore a need for an improved approach in wrought superalloy manufacturing technology to avoid or at least minimize quench cracking.